The invention relates to a method and a device for preparing samples for a chemical analysis method, in particular for chromatography, according to the preamble of the independent patent claims.
As a preparation for carrying out chemical analysis methods such as for example chromatography or flow techniques, the sample to be analysed must be prepared. Analysis samples contain, in addition to the ions or molecules (analytes) to be determined, impurities, particularly larger molecules or solid components which may lead to adulterated analysis results. These impurities must be removed from the analysis sample before the analysis method. Impurities which commonly occur are for example proteins with the analysis of food or filter fibres, dust particles or likewise with analysis in the field of the environment.
For removing these molecules or solid material from the analysis sample, various methods are known. The analysis samples may be prepared by the addition of acid for example and by centrifuging or filtration. From U.S. Pat. No. 5,279,972 and U.S. Pat. No. 4,837,157, separating methods by way of dialysis are known. According to the method of U.S. Pat. No. 4,837,157, the analysis solution and the acceptor solution are moved on both sides of a permeable membrane. The permeable membrane is chosen such that the analytes to be determined may migrate through the membrane, and that the impurities to be excluded from the analysis are kept back by the membrane. A problem with this known method lies in the fact that the acceptor solution containing the analytes may never achieve as high a concentration of analytes as in the original analysis solution. The analytes migrate at a maximum through the membrane into the acceptor solution until an equal concentration on both sides of the membrane is to be established. The concentration of the analytes in the acceptor solution thus comprises at the most, a value of 50%. This is above all disadvantageous then, when various analytes are present in the analysis solution. Ions of different sizes do not migrate with the same speed through the membrane. These theoretical concentration values of 50% are only achieved with very long dialysis times of up to a few hours. In the average case, chloride for example is only enriched to 10% to 30% and sulphate only to 5% to 10% in the acceptor solution. With the occurrence of various analytes there results, because of this, inaccuracies with regard to the ratio of concentrations of the individual analytes. Difficult or only inaccurately reproducable anaylses are the result thereof.
From Martinez et al. (membrane extraction--preconcentration cell coupled on-line to flow injection and liquid chromatographic system. Determination of triazines in oil. Analytica Chimica Acta 304 (1995) 323-332) a membrane extraction is known, in which the acceptor solution is kept still in the acceptor cell for a certain period of time. By way of a directed choice of acceptor solution, the analytes migrated into the acceptor solution are converted (protonized) so that the concentration of unaltered analytes in the acceptor solution constantly remains at a low value. In this manner a gradient is produced which permits the continued diffusion of analytes into the acceptor solution. The disadvantage with this method lies in the fact that to carry out the actual analysis, the converted analytes must be brought back into the original form. This gives rise to an additional method step and increases the danger of losses or measuring errors. Furthermore the method known from Martinez et al cannot easily be applied to the analysis of ions. Whilst Martinez et al procedes from basic organic molecules which are simply removed from equilibrium in a column by protonation, no such simple reactions are possible for ions.